By Jonathan Amos
Science correspondent, BBC News
An artist's impression of the black hole pulling gas off its companion
Astronomers have spied a star-sized black hole much further away than any such object previously known.
It has a mass 20 times that of our Sun and is sited six million light-years away in the galaxy NGC 300.
The discovery was made using the Very Large Telescope (VLT) facility on Mount Paranal in Chile.
The scientists' data indicates the object has a huge companion star that will, most probably, end its days as a black hole, too.
"In the time it's taken for the light to reach us from this galaxy, the companion star will have blown up in a supernova to produce its own black hole," said Professor Paul Crowther, from Sheffield University, UK, and the lead author of the scientific paper reporting the discovery.
"If you could instantly teleport yourself to that system right now, you'd presumable find a pair of black holes spiralling around each other," he told BBC News.
The spiral galaxy NGC 300
You can see a video of what this system looks like by going to the website of the European Southern Observatory organisation.
Black holes tend to come in two sizes. The super-massive variety is colossal and weighs a million to a billion times the mass of our Sun.
There is also the stellar-sized type, which may be 10 or so times the mass of our Sun, and result when really big stars exhaust their nuclear fuel at the end of their lives and collapse.
The new target falls into the latter category. It has a mass of about 20 times that of the Sun.
Astronomers have now found three black holes with masses more than 15 times that of the Sun, all of which are in galaxies outside our own.
The existence of the new one was first suspected through X-ray observations with the US space agency's Swift telescope and the European Space Agency's XMM-Newton observatory.
"We recorded periodic, extremely intense X-ray emission, a clue that a black hole might be lurking in the area," explains team member Stefania Carpano from Esa.
Black holes are expected to pulse X-rays as they pull gas into themselves and tear it apart.
The VLT facility and its four 8.2m telescope units
This was all confirmed when astronomers then followed up the Swift-XMM data with observations using the FORS2 instrument mounted on the 8.2m Antu unit of the VLT.
The instrument can pick apart fine details in light at visible and near-infrared wavelengths.
"We took spectra of the companion star regularly, every night over the course of a couple of weeks, and we saw a feature in the companion star that basically 'wobbles'," said Professor Crowther.
"We get a blue shift and a red shift in the light as the star goes around the black hole."
The pair sweep about each other with a period of just 32 hours. If sited in our Solar System, this action would take place inside what is the orbit of Mercury.
The companion is something astronomers call a Wolf-Rayet star - a giant, hot, highly-evolved star that is billowing gas into space.
SEEKING GRAVITATIONAL WAVES
Gravitational waves are an inevitable consequence of the theory of general relativity
They describe the gravity force as distortions made by matter in the fabric of space-time
An accelerating mass will produce waves; they are expected to propagate at the speed of light
Detectable sources should include exploding stars; merging black holes and neutron stars
Labs fire lasers into 2-4km-long, L-shaped tunnels; the weak waves should disturb the light
A lot of this material is presumably being pulled onto the black hole.
Only one other binary comprising a black hole and a Wolf-Rayet star has previously been seen.
Assuming the Wolf-Rayet arrives at the denouement expected for a star of its size, the system will then become a binary comprising two coalescing black holes.
In time, these holes would merge, emitting copious amounts of energy in the form of gravitational waves.
These ripples in the fabric of space-time are an inevitable consequence of the theory of general relativity, and their first detection is being sought by intricate laser experiments set up in science labs across the world.
Merging black holes are considered one of the most promising targets for these experiments.
"What's not really known is the statistics of binary black holes - we don't know how many there are. This gives us a hint there might be a certain number because this system is a progenitor to binary black holes."
Professor Crowther and colleagues will publish their findings in the journal Monthly Notices of the Royal Astronomical Society.
